Current retardation films are obtained by uniaxially stretching plastic films such as polycarbonate, polyarylate and polyether sulfone and are films having the function of changing the polarization axis of linearly polarized light (optical rotation) and the function of converting linearly polarized light into circularly polarized light or elliptically polarized light. This retardation film generally has so-called wavelength dispersion characteristics that a retardation differs depending on wavelength and the wavelength dispersion characteristics differ depending on the type of material to be used. Usually used retardation films each have the characteristics that the retardation value in the wavelength side longer than 550 nm is smaller than the retardation value at a wavelength of 550 nm and the retardation value in the wavelength side shorter than 550 nm is larger than the retardation value at a wavelength of 550 nm. This tendency is more significant in the shorter wavelength side.
This poses, for example, the problem that when a retardation film (so-called quarter wavelength plate) having a retardation which is to be ¼ of the wavelength is used to produce an anti-reflection filter, a sufficient anti-reflection effect is obtained only in the wavelength range where the retardation is almost ¼ the wavelength and circularly polarized light is converted into elliptically polarized light at other wavelengths with the result that only insufficient anti-reflection effect is obtained. Also, when a retardation film (so-called half-wave plate) having a retardation which is to be ½ of the wavelength is used to produce a rotatory polarizer which is used for a liquid crystal projector and the like, only in the wavelength range where the retardation is almost ½ of the wavelength can rotate a linearly polarized light as linearly polarized light, and linearly polarized light is converted into elliptically polarized light at other wavelengths with the result that only an insufficient rotatory polarizing effect is obtained.
To deal with this problem, a method is proposed in which plural retardation films are laminated with their optical axis being crossed with each other in the publication of Japanese Patent Application Laid-Open (JP-A) No. 5-100114. This method, however, has the problem that it causes an increase in thickness due to the use of plural retardation films, such complexity of the production process in which plural retardation films are laminated with their optical axis being crossed with each other and a reduction in yield (these retardation films must be cut to cross these films with each other). In the meantime, a retardation film is proposed which can impart the same level of a retardation to each wavelength in a wide visible range by using only one film in the publication of JP-A No. 2000-137116.
However, because the wavelength dispersion characteristics are determined by the material to be used in a retardation film, there is a limitation to such a material as described in the publication of JP-A No. 2000-137116 which can improve the wavelength dispersion characteristics by using only one film. Also, because such a material does not function as a retardation film unless it is made into a film and the film is further stretched uniaxially, such a complex step involving filming and stretching must be carried out. Further, in the case where it is necessary to laminate the retardation films such that the polarization axis of a polarization film crosses at a specific angle with the slow axis of a retardation film, it is necessary to cut the film to laminate the cut film on another film even if the retardation film is one film, with the result that this causes a reduction in yield. In order to solve such a problem, there is a method disclosed in the publication of JP-A No. 2000-98133 in which using a composition obtained by adding a mixture of specific nonionic fluoroalkyl-alkoxylate surfactants to a mixture of specific low-molecular liquid crystal compounds which can be oriented on a substrate which has been subjected to rubbing treatment, the liquid crystal compound is oriented in a specific direction to obtain a retardation film. It is however difficult to obtain a very satisfactory film having such wavelength dispersion characteristics that the same level of retardation can be given to each wavelength in a wide visible range. According to the publication of WO02/093213 (PCT/JP02/04523), a retardation film having such wavelength dispersion characteristics is obtained by rubbing or stretching a film obtained from a liquid crystal cellulose derivative and a non-liquid crystal reactive product. However, there is the problem that when the film obtained from the liquid crystal composition as specifically disclosed in the publication is oriented in a specific direction on a rubbed substrate, satisfactory orientation is not obtained.